Wilding, Martin Charles; Navrotsky, Alexandra

Abstract

High temperature calorimetry using molten silicate solvents has been used to evaluate, in situ, the mixing properties of two network-forming oxides Al2O3 and SiO2 in several silicate melts at 1760 K. In K2O-SiO2 solvents, the partial molar enthalpy of mixing of supercooled liquid SiO2 ranges from -11.4 to + 4.6 kJ/mol as SiO2 concentration is increased from 66 to 75 mole %. For mixed cation solvents (0.2CaO, 0.8 K2O)-SiO2, the partial molar enthalpy of mixing ranges from -10.6 to -3.6 kJ/mol for the same composition range. The energetics reflect the depolymerization of the silicate melt framework and the formation of melt species with Si-O-Mn + bonds. Decreased field strength of the network modifying cation, Mn+ favors Si-O-Mn+ formation. However, as predicted by Le Chatelier's Principle, since Si-O-Mn+ formation is exothermic, this species becomes less dominant as temperature is increased. This temperature-dependence of the depolymerizing reactions, which define silicate melt structure, may relate to the configurational changes of alkali silicate melts which result in non-Arrhenian structural relaxation. The enthalpy of solution of Al2O3 depends on the composition of the silicate solvent and the concentration of Al2O3. The enthalpy of solution ranges from 33.3 to 1517 kJ/mol for the concentration range 0 to 10 mole % Al2O3 in potassium silicate solvents. In mixed cation solvents, the enthalpy of solution ranges from 33.6 to 64.4 kJ/mol in the concentration range 0 to 10 mole % Al2O3. The dissolution of Al2O3 in these silicate solvents reflects the interaction between supercooled Al2O3 liquid and the existing melt species and is also influenced by the identity of the charge-balancing cation. In potassium end-member melts, the low degree of interaction between K+ and the polymerized framework means that Al3+ cation repulsions dominate the melt energetics and results in an ordered melt with predominance of K-O-Al species and a strong concentration dependence of the enthalpy of solution. In mixed cation melts the ability of Ca2+ to perturb the melt network results in a melt that has a variety of structural environments for AlO4 tetrahedra. Accordingly the concentration dependence of the heat of solution is less pronounced.